A need for a technique for performing large-volume optical communication at higher speeds has increased as the volume of information transmission has become larger. A technique for performing large-volume optical communication at high speeds is, for example, multi-level modulation, polarization multiplexing, and digital coherent receiving.
When multi-bit information is thus transmitted in one symbol time, characteristics may vary among transmission lanes. In particular, in a case where polarization multiplexing is used, characteristics may vary widely between a transmission lane for an x-polarized wave and a transmission lane for a y-polarized wave because of the PDL (Polarization Dependent Loss) of an optical transmitter, an optical receiver, and a transmission path.
Then, such variation in characteristics among transmission lanes decreases the randomness of received signals, and consequently, deteriorates an error correction performance.
A known optical transmission system which considers variation in characteristics among transmission channels caused by, for example, the PDL of a transmission path and optical devices is one disclosed in Patent Document 1. Patent Document 1 discloses a transmission system which exchanges bits to be transmitted through a channel where information is transmitted by an x-polarized wave and bits to be transmitted through a channel where information is transmitted by a y-polarized wave at a predetermined ratio and thereby assigns transmission data to the respective transmission channels so that variation in characteristics is averaged in all of the transmission channels.    Patent Document 1: Japanese Unexamined Patent Application Publication No. JP-A 2009-089194
However, the technique disclosed in Patent Document 1 cannot sufficiently deal with deterioration of the performance of an error correction code because there is no change in the performance of an error correction code. Thus, there is a problem that it is impossible to sufficiently deal with deterioration of the performance of an error correction code.